Heretofore, the development of polymeric containers for the storage of food stuffs and the like has been a focus in the art. Some of the more popular products of this sort which have been developed for the packaging of food and beverages include polyester-coated paper board for ovenable trays, soft drink carbonated beverage bottles, mouthwash containers, thermoformed blister packs for the packaging of cold cuts, and films in various food wrap applications.
Plastics previously utilized for hot-fill applications have generally been subject to one or more shortcomings such as lack of acceptable strength, the necessity of using special processing techniques entailing a substantial increase in manufacturing costs, or the requirement of using materials which are too expensive to be cost-competitive. Particularly as to the use of polyester and hot-fill food applications, it has been observed that there are two principal problems. Those are (a) if biaxially oriented, polyester containers undergo considerable shrinkage when heated to temperatures at or near their glass transition temperatures, and (b) unoriented polyester containers undergo a crystalline transition at elevated temperatures used in hot-fill food applications causing the containers to become opaque and brittle.
One alternative which has been offered as a possible solution is the making of hot-fill containers of polyethylene terephthalate ("PET"). This technique typically includes the steps of heating an amorphous sheet of unoriented PET, drawing the sheet with a male plug to obtain biaxial orientation in the PET material, transferring the drawn sheet from the male plug to a heated female cavity for the purpose of molding and heat setting the PET sheet, i.e., imparting a degree of crystallinity desired in view of the intended end use. The PET is then allowed to shrink back onto the male plug for cooling to a temperature below its glass transition temperature. See U.S. Pat. No. 4,388,356 granted to Hrivnak, et al. on Jun. 14, 1983. It is said in that patent that the following of the aforementioned technique permits production of a clear and non-brittle article of PET.
However, the technique mentioned in the preceding paragraph is also subject to serious shortcomings. As is clear from the above-identified patent, the amorphous PET sheet utilized is theretofore produced at elevated temperature by extrusion and calendaring. The PET sheet is conventionally allowed to cool and it is presumably stored (e.g. in inventory) until such time as it is to be used in the production of a PET container. In order to carry out the patented process the sheet is then reheated, for instance by indexing through a temperature-controlled oven, to bring it into the thermoformable plastic state. Only after reheating of the PET sheet is it then molded and further heated to convert the amorphous PET to a crystalline state; the degree of crystallinity is the result of the correlative effects of time and temperature (although it should be noted that by conventional inclusion of a nucleating agent in the PET, e.g., 2-3 wt. % of an olefin, the time-at-temperature necessary for crystallization is significantly reduced). Problems in implementing this technology stem from the fact that it entails a first heating of the PET material to render it extrudable, and then a reheating of extruded PET at the time of its formation into a container or the like. That is to say, the conventional technique is disadvantageous due to the significant energy wastage, and thus increased cost, which is inherent in the reheating of the PET to render it thermoformable for molding. The conventional technique is further disadvantageous in that reheating requires impractically difficult control of reheating oven temperatures in order to avoid overheating of the PET sheet before mold contact. Such overheating is undesirable because it introduces a degree of initial crystallization prior to heat setting than can be tolerated, for the reason that excessive crystallization unacceptably reduces the impact resistance of the ultimately thermoformed PET product. Additionally, it would seem that in using the technique claimed in the patent--forcing PET into a female cavity with a male plug--it would be difficult to control the respective dimensions of the male plug and female cavity during heating, thereby leading to likely scraping or rubbing of the plug against the molded PET (due to the close tolerances required) in all but the smallest molding cavity arrangements. These are significant drawbacks.
Accordingly, prior developments leave much to be desired.